Magnetization switching in polycrystalline Mn3Sn thin film induced by self-generated spin-polarized current

Electrical manipulation of spins is essential to design state-of-the-art spintronic devices and commonly relies on the spin current injected from a second heavy-metal material. The fact that chiral antiferromagnets produce spin current inspires us to explore the magnetization switching of chiral spins using self-generated spin torque. Here, we demonstrate the electric switching of noncollinear antiferromagnetic state in Mn3Sn by observing a crossover from conventional spin-orbit torque to the self-generated spin torque when increasing the MgO thickness in Ta/MgO/Mn3Sn polycrystalline films. The spin current injection from the Ta layer can be controlled and even blocked by varying the MgO thickness, but the switching sustains even at a large MgO thickness. Furthermore, the switching polarity reverses when the MgO thickness exceeds around 3 nm, which cannot be explained by the spin-orbit torque scenario due to spin current injection from the Ta layer. Evident current-induced switching is also observed in MgO/Mn3Sn and Ti/Mn3Sn bilayers, where external injection of spin Hall current to Mn3Sn is negligible. The inter-grain spin-transfer torque induced by spin-polarized current explains the experimental observations. Our findings provide an alternative pathway for electrical manipulation of non-collinear antiferromagnetic state without resorting to the conventional bilayer structure.

We also performed XRD measurements for MgO/Mn3Sn(12) and Ti/Mn3Sn(12), where the Mn3Sn thickness is same as the one used for most current-induced switching in this study.
As can be seen from Supplementary Fig. 1e and 1f, MgO/Mn3Sn(12) shows obvious peaks of (112 ̅ 0), which is missing in Ti/Mn3Sn(12), while the peak of (202 ̅ 1) exists in both samples. In addition, there is also a very small peak of (202 ̅ 0) in both samples and a small peak of (101 ̅ 1) in Ti/Mn3Sn(12). This indicates that there are more grains with kagome plane perpendicular to the film plane in MgO/Mn3Sn(12), while the crystalline plane of the grains in Ti/Mn3Sn(12) is mostly tilted towards the film plane.

S2. Fitting and decomposition of M-H curves
We find that the M-H loops of polycrystalline Mn3Sn can be decomposed into sub-loops using the relation        The switching ratio is also extracted, which is shown in Supplementary Fig. 4f. We can find that the critical current density increases monotonically as the decrease of Mn3Sn thickness, but the change is not very significant, which is around the level of 3 MA cm -2 in Ta layer. The larger critical current density at smaller tMn3Sn might be due to the increased switching ratio. As a control, AHE in Ta/Mn3Sn was also studied. Supplementary Fig. 5a   10 assistive field of +400 Oe and -400 Oe are shown in Supplementary Fig. 7b and 7c, respectively. As can be seen, the switching amplitude follows a similar trend as the AHE amplitude, which keeps increasing as the temperature decreases from 400 K to 240 K below which it quickly decreases to nearly zero from 240 K to 200 K. Ta

S5. MOKE images of Mn3Sn at different injection current
Supplementary Fig. 8a and 8b show the current-induced switching process in Mn3Sn as the current is swept from 0 mA to 26 mA at a of +460 Oe and -460 Oe, respectively. The current pulse width is fixed at 5 ms. A negative current of -26 mA was first applied to initiate the magnetic state in Mn3Sn. From the MOKE imaging, the switching starts to occur around +17.5 mA, which is consistent with electrical measurement result shown in Fig. 2b-c of the main text.
As the current further increases from +17.5 mA, the colour of the Hall bar becomes darker at = +460 Oe (Supplementary Fig. 8a) and brighter at = -460 Oe (Supplementary Fig. 8b).
In the whole switching process till it saturates at +26 mA, the change in colour is gradual and homogeneous without clear domain wall propagation observed. Therefore, the switching in polycrystalline Mn3Sn occurs in a gradual rotation manner at individual crystalline grains.

S7. Temperature dependence of resistivity and AHE of Mn3Sn with different seed layers
To gain some insight of the seed layer effect on its crystalline orientations, we also examined the temperature dependence of longitudinal resistivity and Hall conductivity of 12 nm Mn3Sn with different seed layers (MgO and Ti underlayers with different thickness tTi and tMgO).
Supplementary Fig. 10a and 10b   MgO/Mn3Sn might be due to the existence of more group i) grains, whose kagome planes are parallel to z-axis (the direction of applied field in AHE measurement). This corroborates well with the XRD and M-H measurement results shown in Supplementary Information S1 and S2.
From the temperature dependent AHE, we also find that MgO/Mn3Sn still shows negative AHE at low temperature (50 K to 108 K) as shown in Supplementary Fig. 10d, while Ti/Mn3Sn shows ferromagnetic type of AHE ( Supplementary Fig. 10e-f), suggesting the Mn3Sn transition into glassy FM phase 6 . As the increase of Ti thickness, the amplitude of FM-phase AHE at low temperature further increases. In addition, as shown in Supplementary Fig. 10g-

S8. Current-induced switching of Ti/Mn3Sn and Ti/Mn3Sn/Ti
As mentioned in the main text, it is unlikely that there is significant spin-current generation or spin accumulation at Ti/Mn3Sn interface. Nevertheless, to exclude the interface effect at AHE becomes negligibly small and almost submerged by ordinary Hall effect signal (as shown in Supplementary Fig. 11b), which might be because the Ti layer facilitates the formation of (0001) crystalline plane (with c-axis along z-direction) in Mn3Sn, especially a thick Ti layer, as has been discussed in Supplementary S7. Therefore, we could only perform current-induced switching for Ti(1)/Mn3Sn(12). From Supplementary Fig. 11d-e, evident switching is also shown in Ti(1)/Mn3Sn(12), with comparable switching ratio (~18%) and switching current (~20 mA) to the Ti(2)/Mn3Sn(12) case. Therefore, decreasing the Ti thickness does not lead to significant increase of switching ratio or decrease of switching current. Furthermore, for Ti(1)/Mn3Sn(12)/Ti(1) sample with more symmetric structure than Ti/Mn3Sn, the switching (as displayed in Supplementary Fig. 11g-h) also shows same polarity, similar level of switching ratio (~21%) and switching current (~20 mA) as the Ti (1, Oe, respectively.

S9. Current-induced switching at different in-plane assistant field angle
We also performed current-induced switching under an in-plane field with varying field-angle H and a fixed amplitude of 400 Oe ( H = 0 corresponds to ). Supplementary Fig. 12a shows the change in Hall resistance ∆ H in current-induced switching as a function of H for Ta(2)/MgO(2)/Mn3Sn(8). As can be seen, the maximum switching amplitude occurs at 0° (+x) and 180° (-x), which resembles the SOT-induced switching case. However, differently, we also observed switching at 90°(+y) and 270°(-y) as shown in Supplementary Fig. 12b, though the switching amplitude is only one tenth of the value at H = 0. These results further support IGSTT-based magnetization switching in polycrystalline Mn3Sn.

S10. Correlation of switching ratio with AHE remanence
As switching only occurs when the spin polarization is within the kagome plane, it is not possible to achieve 100% switching in polycrystalline samples as revealed by previous studies including the recent study using the nitrogen-vacancy sensor 2 . For HM/Mn3Sn bilayer, where the spin current is mainly from heavy metal layer and the spin polarization direction is fixed, the more kagome planes that are parallel to the spin polarization the larger the switching ratio will be. For simplicity, we assume that the sample consists of group i) grains with randomly distributed orientations. In this case, we may estimate the switching ratio of HM/Mn3Sn bilayers by assuming 1) the switching is based on conventional damping-like SOT, 2) only grains with crystalline plane coplanar with the spin polarization can be switched, and 3) grains which meet the polarization requirement are completely switched at sufficiently large current.
Therefore, the estimated maximum switching ratio turned out to be 2 ∫ cos /2 0 = 0.64.
Here, is the misalignment angle of c-axis of kagome plane and spin polarization from HM metal layer. In fact, the maximum switching ratio for HM/Mn3Sn bilayer structure is around 50% in reported study 7 and 61% in the our present work, which is close to the estimated value.
The smaller switching ratio in previous studies might be due to the larger Mn3Sn thickness used (30-40 nm). On the other hand, when the switching is mainly induced by the self-generated spin current in Mn3Sn, the switching ratio can be much smaller because a pair of neighbouring grains functioning as polarizer and analyzer with specific crystalline orientation is required.
From current-induced switching in different structures, we find that the switching ratio overall presents a positive correlation with the AHE remanence H( =0) / AHE as shown in Supplementary Fig. 13, which is an indicator of degree of texturing of the non-collinear AFM film. The largest switching ratio achieved is around 20% in Ti/Mn3Sn, where the AHE remanence is also largest. Nevertheless, we believe 100% switching is possible for small samples with a single grain in Ta